In an active-matrix display device such as a liquid crystal display, a thin-film transistor device referred to as a thin-film transistor (TFT) has been used. In the display device, the TFT is used as a switching device for selecting a pixel or a driving transistor for driving the pixel.
In recent years, organic EL displays using electroluminescence (EL) of an organic material have been attracting attention as a type of next-generation flat panel display replacing liquid crystal displays.
Unlike the voltage-driven liquid crystal display, the organic EL display is a current-driven display device. Accordingly, there is an urgent need for development of a thin-film transistor device having excellent on/off-characteristics as a driving circuit for the active-matrix display device. A TFT has a structure in which a gate electrode, a semiconductor layer (channel layer), a source electrode, and a drain electrode are formed on a substrate. The channel layer is typically a silicon thin film.
Moreover, display devices are demanded to increase a screen size and reduce a cost. In general, because of easy cost reduction, bottom-gate TFTs are used. A bottom-gate TFT has a gate electrode formed closer to a substrate more than a channel layer is.
The bottom-gate TFTs are classified into two major categories: channel-etching TFTs in which a channel layer is etched, and channel-stopper TFTs in which a channel layer is prevented from being etched.
In comparison to the channel-stopper TFTs, the channel-etching TFTs can decrease steps of photolithography, thereby reducing a manufacturing cost.
For example, Patent Literature 1 discloses a channel-etching thin-film transistor capable of increasing On-current and suppressing Off-current. The thin-film transistor disclosed in Patent Literature 1 includes: a gate electrode on a substrate; a gate insulating film on the gate electrode; an active layer composed of a polycrystalline silicon film on the gate insulating film; contact layers each provided between the active layer and a corresponding one of the source electrode and the drain electrode; an electric field limiting layer composed of an amorphous silicon film provided between the contact layers and the active layer.